A combustion exhaust gas coming from a thermoelectric power station, a municipal refuse incinerator or an industrial waste incinerator is mainly composed of moisture, sulfur oxide (sulfur dioxide, sulfur trioxide), hydrogen chloride, nitrogen oxide, carbon dioxide, nitrogen and oxygen. When sulfur trioxide is included in a combustion exhaust gas even at 1 ppm, in particular, the dew point of the exhaust gas reaches a temperature of 100° C. or higher and thus sulfuric acid dew-point corrosion is caused. To cope with the corrosion problem, a sulfuric acid dew-point corrosion resistant steel (for example, Japanese Examined Patent Publication No. S43-14585) and a highly corrosion resistant stainless steel (for example, Japanese Unexamined Patent Publication No. H7-316745) are used.
In the case of the flue gas system of a coal burning thermal power boiler or a non-industrial or industrial waste incinerator, as a considerable amount of hydrogen chloride is contained, besides the aforementioned sulfur trioxide, in combustion gas, both sulfuric acid condensation and hydrochloric acid condensation are caused. Further, a sulfuric acid dew-point temperature and a hydrochloric acid dew-point temperature vary in accordance with the composition of the combustion exhaust gas. In general, a sulfuric acid dew-point temperature is roughly in the range from 100° C. to 150° C. and a hydrochloric acid dew-point temperature is in the range from 50° C. to 80° C. Therefore, in a flue gas system, even though the configuration of the system is not changed and the temperature of a passing combustion gas is nearly constant, some portions are dominated by sulfuric acid dew-point corrosion and some other portions by hydrochloric acid dew-point corrosion because the temperatures of the walls of the portions vary in accordance with the structures and the portions. For that reason, the development of a material for a structural member excellent in both sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance has been a challenge.
As an example to cope with the challenge, Japanese Examined Patent Publication No. S46-34772 discloses a technology related to an acid resistant ultra-low carbon low alloy steel, wherein sulfuric acid resistance and hydrochloric acid resistance are enhanced by ultra-low carbon (not more than 0.03 mass % C) and the addition of Cu and Mo.
Further, Japanese Unexamined Patent Publication No. H9-25536 discloses a technology related to a sulfuric acid dew-point corrosion resistant steel, wherein hydrochloric acid dew-point corrosion resistance is improved, while sulfuric acid dew-point corrosion resistance is secured, by reducing an S amount and adding Sn and/or Sb. Furthermore, Japanese Unexamined Patent Publication No. H10-110237 discloses a steel excellent in sulfuric acid resistance, hydrochloric acid resistance and hot-workability, wherein Sn and/or Sb are added to a copper contained steel of ultra-low sulfur (not more than 0.005 mass % S) and further Mo and/or B are added thereto for recovering hot-workability that has been deteriorated by the addition of Sn and/or Sb.
In the case of the technology disclosed in Japanese Examined Patent Publication No. S46-34772, however, the steel is limited to an ultra-low carbon steel and therefore there have been the problems of requiring other expensive alloying elements for enhancing strength, limiting the application of the steel to a heat conduction element plate for an air heater or the like to which even a steel having the tensile strength of less than 400 N/mm2 is applicable, and therefore being hardly used as a steel for a welded structure. Further, though the details are explained later, there have been the problems of: not only extremely hindering sulfuric acid resistance in the sulfuric acid concentration range of 10 to 40 mass % when Mo is added excessively within the component range specified in the publication; but also, under some conditions, deteriorating hydrochloric acid resistance to worse than that of a steel according to Japanese Examined Patent Publication No. S43-14585 or an S-TEN 1 steel (a Cu—Sb steel) (Nippon Steel Corporation Product Catalog, sulfuric acid dew-point corrosion resistant steel S-TEN, Cat. No. AC107, 1981, Sixth Edition).
In the case of a sulfuric acid dew-point corrosion resistant stainless steel disclosed in Japanese Unexamined Patent Publication No. H7-316745 and others, there have been the problems of: requiring securing resistance to stress corrosion cracking and crevice corrosion besides general corrosion resistance because stress corrosion cracking and crevice corrosion, together with dew-point corrosion, are caused by the concentration of chloride in a hydrochloric acid dew-point corrosion environment; therefore inevitably requiring an increase in alloying elements; causing the cost to increase further than that of a low alloy steel and strength to increase; and therefore making it difficult to secure both acid-resistance and cold-workability simultaneously.
Further, in the case of a steel disclosed in Japanese Unexamined Patent Publication No. H9-25536, there has been the problem of only showing sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance equal to or inferior to the hydrochloric acid dew-point corrosion resistance of aforementioned S-TEN 1.
Furthermore, in the case of a steel disclosed in Japanese Unexamined Patent Publication No. H10-110237, firstly, there has been the problem of incurring excessive increase of a steelmaking cost because the steel requires ultra-low sulfur. A second problem of the steel, which has newly been found by the present inventors, is that an excessive reduction of sulfur deteriorates sulfuric acid resistance, as mentioned later. A third problem thereof has been that, when the steel is used for a welded structure, though a weld metal also requires ultra-low sulfur at the same level as the steel, it is extremely difficult to attain such ultra-low sulfur when some other requirements such as weld operability are taken into consideration and, therefore, when the steel is used for a welded structure, namely a join, the corrosion resistance of the join is hardly secured. A fourth problem thereof has been that, though the addition of Sn and/or Sb is inevitably required, an excessive addition of Sn and/or Sb conspicuously deteriorates the toughness of a steel sheet as mentioned later and, therefore, the application of the steel to a hot-rolled steel sheet for a welded structure such as a duct or a stack is not practical.
As a result of the study by the present inventors, it has been clarified that the aforementioned S-TEN 1 steel designed by employing the composition of a Cu—Sb steel as the basic composition has the most excellent hydrochloric acid resistance among various sulfuric acid dew-point corrosion resistant steels that have ever been developed.
However, in some waste incinerators for burning vinyl chlorides and kitchen garbage, the concentration of hydrogen chloride in an exhaust gas may sometimes reach 4,000 ppm. For that reason, a novel sulfuric acid dew-point corrosion resistant steel, that: has outstandingly improved hydrochloric acid resistance while the sulfuric acid resistance of an S-TEN steel is maintained; can be produced as a steel for a welded structure and a cold-rolled steel sheet excellent in cold-workability; and is far more economical than a highly corrosion resistant stainless steel excellent in hydrochloric acid resistance, has strongly been desired.
Further, such a low alloy steel is often used for an ordinary welded structure.
In general, in the case where a welded structure is used in a corrosive environment, when the corrosion resistance of a weld differs from that of a base material, the one having inferior corrosion resistance is selectively corroded and the service life of the welded structure is extremely shortened. What is worse, when a weld is selectively corroded, stress concentration occurs at corroded pores and, in the extreme case, the destruction of a structure may be caused. Therefore, when such a low alloy steel is used for a welded structure that cannot ignore corrosion degradation, the corrosion resistance of not only a base material but also a weld must be secured sufficiently.
In smoke exhausting equipment such as a flue and a stack of a coal burning thermal power boiler or a refuse incinerator, sulfuric acid dew-point corrosion and hydrochloric acid dew-point corrosion are caused by sulfur trioxide and hydrogen chloride in an exhaust gas. In such an environment, a sulfuric acid dew-point corrosion resistant steel (refer to, for example, Nippon Steel Corporation, S-TEN Product Catalog, Cat. No. AC107, 1981, Sixth Edition) is used. As a welding material, a welding material for a soft steel and a welding material exclusively used for a sulfuric acid dew-point corrosion resistant steel (refer to, for example, Nippon Steel Welding Products & Engineering Co., LTD., Nittetsu welding Material & Apparatus Handbook, p. 61, p. 164, p. 208 and p. 291) are used.
A welding material exclusively used for a sulfuric acid dew-point corrosion resistant steel has been a material containing Cu itself as a corrosion resistant element or a material containing components of a Cu and Cr. A weld joint formed by using such an existing welding material shows a sufficiently excellent corrosion resistance in a sulfuric acid dew-point corrosion environment caused by the smoke exhaustion equipment of a single-fuel fired boiler for heavy oil. However, the problem of such a weld joint has been that, in an environment of a coal burning boiler, a refuse incinerator, a gasifying fusion furnace or the like, both sulfuric acid dew-point corrosion and hydrochloric acid dew-point corrosion are caused simultaneously, therefore the corrosion resistance of the weld metal at the weld is insufficient, and the weld metal that has corrosion resistance inferior to that of the base material is selectively corroded.
The present invention is intended to solve the above problems and the object thereof is to provide: a low alloy steel and a weld joint thereof excellent in acid dew-point corrosion resistance; specifically, a low alloy steel and a weld joint thereof excellent in resistance to hydrochloric acid and sulfuric acid; and more specifically, a low alloy steel and a weld joint thereof which secure an excellent resistance to sulfuric acid dew-point corrosion and hydrochloric acid dew-point corrosion caused in an atmosphere of a combustion exhaust gas coming mainly from a boiler or a gasifying fusion furnace.
To be more precise, the object of the present invention is to provide a low alloy steel and a weld joint thereof which are used for smoke exhaustion equipment of a boiler wherein fossil fuel such as heavy oil and coal, fuel gas such as liquefied natural gas, non-industrial waste such as municipal refuse, industrial waste such as wood chips, fiber chips, waste oil, plastics, waste tires and medical waste, and sewage sludge are incinerated, the smoke exhausting equipment including a gas duct, a casing, a heat exchanger, a gas-to-gas heater, a wet or dry desulfurizer, an electrostatic precipitator, an induced draft fan, a basket material and a heat conduction element plate of a rotary regenerative air preheater, a cooling tower, a bag filter and a stack. To be still more precise, the object of the present invention is to economically provide: an acid dew-point corrosion resistant low alloy steel and a weld joint thereof excellent in economical efficiency, which show an excellent resistance to sulfuric acid or hydrochloric acid dew-point corrosion caused in smoke exhaustion equipment and are applicable to a welded structure and also to a fin of a heat exchanger tube, a heat conduction element plate of an air preheater and an expansion joint of a flue requiring severe cold-workability; and a sulfuric acid dew-point corrosion resistant low alloy steel and a weld joint thereof excellent in corrosion resistance to hydrochloric acid, which are applicable to a steel pickling tank for receiving pickling liquor composed of either hydrochloric acid or sulfuric acid or a mixture thereof.